Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Each pixel incorporates several, differently colored light-emitting elements commonly referred to as sub-pixels, typically emitting red, green, and blue light, to represent each image element. As used herein, pixels can refer to a single light-emitting element or a group of differently colored light-emitting elements. A variety of flat-panel display technologies are known, for example plasma displays, liquid crystal displays, and light-emitting diode (LED) displays.
Light emitting diodes (LEDs) incorporating thin films of light-emitting materials forming light-emitting elements have many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 to Tang et al. shows an organic LED (OLED) color display that includes an array of organic LED light-emitting elements. Alternatively, inorganic materials can be employed and can include phosphorescent crystals or quantum dots in a polycrystalline semiconductor matrix. Other thin films of organic or inorganic materials can also be employed to control charge injection, transport, or blocking to the light-emitting-thin-film materials, and are known in the art. The materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. Light is emitted from a pixel when current passes through the light-emitting material. The frequency of the emitted light is dependent on the nature of the material used. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both.
LED devices can include a patterned light-emissive layer wherein different materials are employed in the pattern to emit different colors of light when current passes through the materials. Alternatively, one can employ a single emissive layer, for example, a white-light emitter, together with color filters for forming a full-color display, as is taught in U.S. Pat. No. 6,987,355 by Cok. It is also known to employ a white sub-pixel that does not include a color filter, for example, as taught in U.S. Pat. No. 6,919,681 by Cok et al. This and other disclosures teach a design employing an unpatterned white emitter together with a four-color pixel including red, green, and blue color filters and sub-pixels and an unfiltered white sub-pixel to improve the efficiency of the device (see, e.g. U.S. Pat. No. 7,230,594 to Miller, et al).
Two different methods for controlling the pixels in a flat-panel display device are generally known: active-matrix control and passive-matrix control. In a passive-matrix device, the substrate does not include any active electronic elements (e.g. transistors). An array of row electrodes and an orthogonal array of column electrodes in a separate layer are formed over the substrate; the intersections where the row and column electrodes overlap form the electrodes of a light-emitting diode. External driver chips then sequentially supply current to each row (or column) while the orthogonal column (or row) supplies a suitable voltage to illuminate each light-emitting diode in the row (or column). Therefore, a passive-matrix design employs 2n connections to produce n2 separately controllable light-emitting elements. However, a passive-matrix drive device is limited in the number of rows (or columns) that can be included in the device since the sequential nature of the row (or column) driving creates flicker. If too many rows are included, the flicker can become perceptible. Moreover, the currents necessary to drive an entire row (or column) in a display can be problematic and the power required for the non-imaging pre-charge and discharge steps of PM driving become dominant as the area of the PM display grows. These two problems limit the physical size of a passive-matrix display.
In an active-matrix device, active control elements are formed of thin films of semiconductor material, for example amorphous or poly-crystalline silicon, distributed over the flat-panel substrate. Typically, each sub-pixel is controlled by one control element and each control element includes at least one transistor. For example, in a simple active-matrix organic light-emitting (OLED) display, each control element includes two transistors (a select transistor and a drive transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each light-emitting element typically employs an independent control electrode and a common electrode. Control of the light-emitting elements is typically provided through a data signal line, a select signal line, a power connection and a ground connection. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control. The same number of external control lines (except for power and ground) can be employed in an active-matrix device as in a passive-matrix device. However, in an active-matrix device, each light-emitting element has a separate driving connection from a control circuit and is active even when not selected for data deposition so that flicker is eliminated.
One common, prior-art method of forming active-matrix control elements typically deposits thin films of semiconductor materials, such as silicon, onto a glass substrate and then forms the semiconductor materials into transistors and capacitors through photolithographic processes. The thin-film silicon can be either amorphous or polycrystalline. Thin-film transistors (TFTs) made from amorphous or polycrystalline silicon are relatively large and have lower performance compared to conventional transistors made in crystalline silicon wafers. Moreover, such thin-film devices typically exhibit local or large-area non-uniformity across the glass substrate that results in non-uniformity in the electrical performance and visual appearance of display employing such materials.
Employing an alternative control technique, Matsumura et al., in U.S. Patent Application Publication No. 2006/0055864, describe crystalline silicon substrates used for driving LCD displays. The application describes a method for selectively transferring and affixing pixel-control devices made from first semiconductor substrates onto a second planar display substrate. Wiring interconnections within the pixel-control device and connections from busses and control electrodes to the pixel-control device are shown.
Since a conventional passive-matrix display design is limited in size and number of light-emitting elements, and an active-matrix design using TFTs has lower electrical performance, there is a need for improved control for display devices employing LEDs that overcomes these problems.